Ground fault protection for personnel and electrical equipment is an ever increasing concern in both home and industry. For residential circuit applications, ground fault circuit interrupting (GFCI) devices are now widely available in compact circuit breaker and receptacle configurations for convenient installation in existing service entry equipment and well receptacle boxes. These GFCI devices are primarily intended to protect people from the hazards of electrical shock caused by leakage current emanating from ground faults, however they do afford a measure of equipment protection in terms of acting to halt ground fault currents which can be damaging to insulation.
For higher current applications found in industry, ground fault protection is available in basically two configurations. With the advent of so-called "static trip" circuit breakers, it has become economically possible to combine overcurrent and ground fault signal processing circuitry in a compact electronic trip unit package which can be integrated with the circuit breaker to achieve comprehensive circuit protection. Alternatively, traditional circuit breakers thermal-magnetic and dual-magnetic trip units for overcurrent protection can be utilized with so-called "ground fault relays" in providing ground fault protection as well. These relays respond to a ground fault signal developed by a suitable sensor, such as a zero sequence transformer coupled with the load carrying conductors, by energizing (or de-energizing) a solenoid which, in turn trips breaker to initiate circuit interruption.
One industrial application where ground fault protection is particularly important is in the mining industry where local grounding of necessary portable electrical machinery in not practical. Thus, for personnel safety, a low impedance ground at the machinery site must be maintained through a wire grounded at the source and running to the machinery with so-called trailing cable utilized in conveying electrical power to the machinery. It will be appreciated that a discontinuity in this grounding conductor or a degradation of its requisite low impedance character would permit the voltage on the machinery frame, should a breakdown in insulation exist between it and one of the line conductors, to rise above ground potential. With the high voltage and current levels typically utilized in the mining industry, the potential for a lethal shock to machinery operators or explosion due to arcing is high even though the ground fault is cleared promptly upon recognition of the flow of ground leakage current.
It is accordingly an object of the present invention to provide an inexpensive ground fault protection system suitable for high current, industrial circuit applications.
An additional object of the present invention is to provide a ground fault protection system of the above character which utilizes as its fault clearing component a circuit interrupting device of the more traditional, non-static trip design.
A further object of the present invention is to provide a ground fault protection system of the above character which utilizes as one of its principle components a low-cost ground fault circuit interrupting (GFCI) device of the type presently enjoying wide use in low-power residential circuitry.
Yet another object is to provide a ground fault protection system of the above character which is capable of monitoring the continuity of a ground circuit path and operating automatically to interrupt a power distribution circuit should the impedance of the ground fault circuit path exceed a preselected value.
Other objects of the invention will in part be obvious and in part appear hereinafter.